Method and apparatus for quasi-sensorless adaptive control of switched reluctance motor drives

ABSTRACT

A method and apparatus for quasi-sensorless adaptive control of a high rotor pole switched-reluctance motor (HRSRM). The method comprises the steps of: applying a voltage pulse to an inactive phase winding and measuring current response in each inactive winding. Motor index pulses are used for speed calculation and to establish a time base. Slope of the current is continuously monitored which allows the shaft speed to be updated multiple times and to track any change in speed and fix the dwell angle based on the shaft speed. The apparatus for quasi-sensorless control of a high rotor pole switched-reluctance motor (HRSRM) comprises a switched-reluctance motor having a stator and a rotor, a three-phase inverter controlled by a processor connected to the switched-reluctance motor, a load and a converter.

RELATED APPLICATIONS

This application claims priority from the U.S. provisional applicationwith Ser. No. 62/519,807, which was filed on Jun. 14, 2017. Thedisclosure of that provisional application is incorporated herein as ifset out in full.

BACKGROUND OF THE DISCLOSURE Technical Field of the Disclosure

This invention relates generally to switched reluctance motor drivesystems, and more particularly to a system for rotor position estimationbased on the measurement of inductance of the phases of the switchedreluctance motor.

Description of the Related Art

A switched reluctance motor (“SRM”) is a rotating electric machine whereboth stator and rotor have salient poles. The switched reluctance motoris a viable candidate for various motor control applications due to itsrugged and robust construction. The switched reluctance motor is drivenby voltage strokes coupled with a given rotor position. The SRM is abrushless electrical machine with multiple poles on both rotor andstator. The stator has phase windings, unlike the rotor which isunexcited and has no windings or permanent magnets mounted thereon.Rather, the rotor of an SRM is formed of a magnetically permeablematerial, typically iron, which attracts the magnetic flux produced bythe windings on the stator poles when current is flowing through them.The magnetic attraction causes the rotor to rotate when excitation tothe stator phase windings is switched on and off in a sequential fashionin correspondence to rotor position. For an SRM, a pair of diametricallyopposed stator poles produces torque in order to attract a pair ofcorresponding rotor poles into alignment with the stator poles. As aconsequence, this torque produces movement in a rotor of the SRM.

The use of switched reluctance motor drives for industrial applicationsis of recent origin. SRM drives have been considered as a possiblealternative to conventional drives in several variable speed driveapplications. In conventional SRMs, a shaft angle transducer, such as anencoder or a resolver, generates a rotor position signal and acontroller reads this rotor position signal. In an effort to improvereliability while reducing size and cost, various approaches havepreviously been proposed to eliminate the shaft position sensor bydetermining the reference commutation angle. These approaches implementindirect rotor position sensing by monitoring terminal voltages andcurrents of the motor. The performance of a switched reluctance machinedepends, in part, on the accurate timing of phase energization withrespect to rotor position. These methods are useful when at least onephase is energized and the rotor is spinning.

Another approach describes a system and method for achieving sensorlesscontrol of SRM drives using active phase voltage and currentmeasurements. The sensorless system and method generally relies on adynamic model of the SRM drive. Active phase currents are measured inreal-time and, using these measurements, the dynamic equationsrepresenting the active phases are solved through numerical techniquesto obtain rotor position information. The phase inductances arerepresented by a Fourier series with coefficients expressed aspolynomial functions of phase currents to compensate for magneticsaturation. The controller basically runs the observer in parallel withthe drive system. Since the magnetic characteristics of the motor areaccurately represented, the state variables, as computed by theobserver, are expected to match the actual state variables. Thus, rotorposition, which is also a state variable, will be available indirectly.This system teaches the general method for estimating rotor positionusing phase inductance measured from the active phase. Here, they applyvoltage to the active phase and measure the current response to measureposition. This current magnitude is kept low to minimize any negativetorque generated at the shaft of the motor.

Another approach describes a method of indirect motor position sensingthat involves applying voltage sensing pulses to one unenergized phase.The result is a change in phase current which is proportional to theinstantaneous value of the phase inductance. Proper commutation time isdetermined by comparing the change in phase current to a referencecurrent, thereby synchronizing phase excitation to rotor position. Phaseexcitation can be advanced or retarded by decreasing or increasing thethreshold, respectively. Due to the unavailability of inactive phasesduring higher speeds, this commutation method which makes use of theinactive phases of the SRM is limited to low speeds. Furthermore,although current and torque levels are relatively small in an inactivephase, they will contribute to a loss in SRM efficiency in thisapplication.

Yet another approach discloses a rotor position estimator for an SRMbased on instantaneous phase flux and phase current measurements. Phasecurrent and flux sensing are performed for the phases in a predeterminedsequence that depends on the particular quadrant of SRM operation. Foreach phase in the predetermined sequence of sensing, phase flux andphase current measurements are made during operation in a pair ofpredetermined sensing regions, each defined over a range of the rotorpositions. The rotor position estimates are derived from the phase fluxand phase current measurements for each respective phase during therespective sensing regions thereof. The rotor position estimates foreach phase are normalized with respect to a common reference phase, anda rotor position estimate for the SRM is computed according to anequation which accounts for the fact that for any given rotor positiondetermined, the rotor poles of the SRM may be approaching alignment ormisalignment. Sampled phase voltage and phase current are integrated toobtain phase flux.

There remains a need for a method of quasi-sensorless adaptive controlof a switched reluctance motor drive using a unique sequence of relationbetween phase inductances to enhance the accuracy of rotor positionestimation. This method would very tightly monitor the speed with asmany as 30 updates per revolution, which would thus provide a higherresolution than several sensorless approaches currently in use. Such aneeded method would automatically accommodate for motor-to-motor orprocess variations, since it would not assume complete uniformity amongall manufactured machines. Further, this approach would create a controlalgorithm that would not need to be calibrated for all motorspecifications and power ratings. Moreover, this method would be able tonaturally calibrate the control algorithm to the inductance profile ofthe machine that is being tested. Such a system would not require anyadjustment in the control algorithm and would not require any priorknowledge of manufacturing specifications of the motor, which wouldfurther reduce the constructional detail burden of the machinemanufacturer. This approach would use its own set of steps forautomatically calibrating the inductance profile for any machine andwould thus save time and resources involved in setting up and testing ofthe machine in an industry setting. Finally, the method would bereliable, robust, and completely scalable and would provide a cleartechnique that actively seeks to calibrate the model to each machinethat is manufactured. The present embodiment overcomes shortcomings inthe field by accomplishing these critical objectives.

SUMMARY OF THE DISCLOSURE

To minimize the limitations found in the existing systems and methods,and to minimize other limitations that will be apparent upon the readingof this specification, the preferred embodiment of the present inventionprovides a method and apparatus for quasi-sensorless adaptive control ofa high rotor pole switched-reluctance motor (HRSRM).

The method comprises the steps of: estimating an initial position of therotor of the HRSRM using a unique sequence of relation between the phaseinductances of the HRSRM, then aligning the rotor with respect to theinitial position to start with a known phase and to provide rotation inthe correct direction. Current is applied to an active phase winding torotate the motor and estimate position using a diagnostic pulse on aninactive phase winding during the initial rotation by estimating theinductance profile. A voltage pulse is applied to the inactive phasewinding and current response in each inactive phase is measured. Nextthe system applies multiple diagnostic pulses to the inactive phase toidentify the next phase and a firm time base is established for asoftware control module on a magnetic sensor. Next the system calculatesthe speed and updates the time base by generating motor index pulsesfrom the magnetic sensor. The shaft speed of the motor is calibratedwhich in turn calibrates a software encoder to operate on the time base.The method cancels out a switching threshold of the magnetic sensor and,monitors the slope of the current waveform in the active phase tofine-tune a firing angle from the encoder software. The pulse timet_(on) based on the estimated time base and the shaft speed is adjustedto track any monitored change in speed. The dwell angle based on theshaft speed is fixed.

The apparatus for quasi-sensorless control of a high rotor poleswitched-reluctance motor (HRSRM) comprises a switched-reluctance motorhaving a stator and a rotor, a three-phase inverter controlled by aprocessor connected to the switched-reluctance motor, a load and aconverter. The rotor includes a plurality of circumferentially spacedrotor poles and rotationally related to a motor shaft having a magneticsensor. The three-phase inverter is adaptable to act as a power supplyto the switched-reluctance motor, the processor having a softwarecontrol module and a software encoder. The load is connected to theswitched-reluctance motor via an inline torque meter and the converterconnected to the load.

It is a first objective of the present invention to provide a method forquasi-sensorless adaptive control of switched reluctance motor drivethat employs a unique sequence of relation between phase inductances toenhance the accuracy of rotor position estimation.

A second objective of the present invention is to provide a method thatmonitors the calculated shaft speed and continuously updates if anychange in speed is detected.

A third objective of the present invention is to provide a method thatcreates a control algorithm that does require calibration for all motorspecifications and power ratings.

A fourth objective of the present invention is to provide a method thatnaturally calibrates the control algorithm to the inductance profile ofthe machine that is being tested.

Another objective of the present invention is to provide a method andapparatus that does not require any adjustment in the control algorithmor any prior knowledge of manufacturing specifications of the motor,which eliminates the constructional details needed from the machinemanufacturer.

Yet another objective of the present invention is to provide a methodthat automatically calibrates the inductance profile for any machine andthus saves time and resources in the characterization and testing of themachine in an industry setting.

Still another objective of the present invention is to provide a methodthat is reliable, robust, and scalable and provides a clear techniquethat actively seeks to calibrate the model to each machine that ismanufactured.

These and other advantages and features of the present invention aredescribed with specificity so as to make the present inventionunderstandable to one of ordinary skill in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to enhance their clarity and improve understanding of thesevarious elements and embodiments of the invention, elements in thefigures have not necessarily been drawn to scale. Furthermore, elementsthat are known to be common and well understood to those in the industryare not depicted in order to provide a clear view of the variousembodiments of the invention, thus the drawings are generalized in formin the interest of clarity and conciseness.

FIG. 1 illustrates a flow chart of a method for controlling a high rotorpole switched-reluctance motor (HRSRM) in accordance with the presentinvention;

FIG. 2 is a graph illustrating an inductance profile according to thephase changes of a three phase SRM in accordance with the presentinvention;

FIG. 3 illustrates a block diagram of an apparatus for controlling thehigh rotor pole switched-reluctance motor (HRSRM) in accordance with thepresent invention;

FIG. 4A is a graph illustrating a current waveform of a three-phase SRMat a particular load in accordance with the present invention;

FIG. 4B is a graph illustrating the current waveform of the three-phaseSRM at another load in accordance with the present invention; and

FIG. 4C is a graph illustrating the current waveform of the three-phaseSRM at yet another load in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following discussion that addresses a number of embodiments andapplications of the present invention, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. It is to be understood that other embodiments may be utilizedand changes may be made without departing from the scope of the presentinvention.

Various inventive features are described below that can each be usedindependently of one another or in combination with other features.However, any single inventive feature may not address any of theproblems discussed above or only address one of the problems discussedabove. Further, one or more of the problems discussed above may not befully addressed by any of the features described below.

As used herein, the singular forms “a”, “an” and “the” include pluralreferents unless the context clearly dictates otherwise. “And” as usedherein is interchangeably used with “or” unless expressly statedotherwise. As used herein, the term ‘about” means +/−5% of the recitedparameter. All embodiments of any aspect of the invention can be used incombination, unless the context clearly dictates otherwise.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”. Words using the singular or pluralnumber also include the plural and singular number, respectively.Additionally, the words “herein,” “wherein”, “whereas”, “above,” and“below” and words of similar import, when used in this application,shall refer to this application as a whole and not to any particularportions of the application.

The description of embodiments of the disclosure is not intended to beexhaustive or to limit the disclosure to the precise form disclosed.While the specific embodiments of, and examples for, the disclosure aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the disclosure, as thoseskilled in the relevant art will recognize.

Referring to FIGS. 1-2, a flow chart of a method for controlling a highrotor pole switched-reluctance motor (HRSRM) 100 in accordance withpresent invention is illustrated in FIG. 1. The method 100 described inthe present embodiment enhances the accuracy of estimating the rotorposition and also allows positioning of at least one phase of the SRM atthe fully aligned position from either rotating in a clockwise orcounterclockwise direction. The method 100 allows quasi-sensorlesscontrol of speed in the switched reluctance motor and creates a controlalgorithm that naturally calibrates with the inductance profile of theSRM. For the proper switching operation of the SRM, it is important tosynchronize the stator phase excitation with the rotor position.

The method 100 comprises the steps of: estimating an initial position ofthe rotor of the HRSRM using a unique sequence of relation between thephase inductances of the HRSRM as indicated at block 102.

The rotor produces an inductance profile in each of the stator poles aseach of the rotor poles comes into and out of alignment with the statorpoles when the rotor is rotated. The inductance profile of a three phaseSRM is illustrated in FIG. 2. For example, to estimate the initial rotorposition, six startup regions are defined in the inductance profile asillustrated in FIG. 2 where the values of the phase inductances have afixed relationship. Let L_(a), L_(b) and L_(c) be the inductances ofphases A, B and C respectively. From the inductance relationship, whichphase or phases needs to be excited to drive the motor to a fullyaligned position can be identified. The initial position is determinedby applying a voltage pulse to each phase winding in turn and measuringthe time it takes for the resulting phase current to reach a presetlimit.

To determine the initial position, a voltage pulse is applied to eachphase winding in turn and the time it takes for the resulting phasecurrent to reach a preset limit is measured. The current ramp time is afunction of the phase inductance and voltage pulse amplitude and isgiven by the following equation:E=L*di/dtwhere E is the applied voltage reference amplitude, L is the phaseinductance and i is the phase current.

The time for the current to rise to the reference limit is longer thegreater the value of the inductance. For an initial phase current ofzero and a reference current of Iref, the time Tp to reference is givenby;Tp=L*Iref/EInitial position is identified from the measured current ramp time.

Based on the initial position, hard alignment is set so as to start witha known phase and to rotate in the correct direction as indicated atblock 104. Current is applied to an active phase winding to rotate themotor as indicated at block 106. The active phase is essentially thephase that has a rotor pole nearest the closest aligned position. Duringthe initial rotation, position is estimated by applying a diagnosticpulse on an inactive phase winding and by estimating the inductanceprofile as indicated at block 108. The inductance profile of SRMindicates that inductance is at a maximum when the rotor is in analigned position and minimum inductance occurs when the rotor is in anunaligned position. The next step is the application of a voltage pulseto the inactive phase winding and measurement of current response ineach inactive phase as indicated at block 110. Multiple diagnosticpulses are applied to the inactive phase to identify when the next phasemust be fired as indicated at block 112 and establishing a firm timebase for a software control module on a magnetic sensor as indicated atblock 114. The magnetic sensor generates index pulses from the magneticsensor (20 edges per revolution) to calculate speed and continuouslyupdate the time base. Multiple diagnostic pulses can be applied for 10rotations to establish a firm time base for the software control moduleto fire the next phase. After 10 rotations, the software timing takesover and the inactive phase is no longer necessary to maintainoperation. The motor speed is calculated and the time base is updated bygenerating motor index pulses from the magnetic sensor as indicated atblock 116. Three signals are generated per rotor pole. In other words,shaft speed for the motor is calibrated 30 times for a motor with 10rotor poles. This step is repeated for 10 (or more, for higher accuracy)revolutions and is used to calibrate a software encoder to operate onthis time base. As indicated at block 118, the shaft speed of the motoris calibrated and the software encoder is calibrated to operate on thetime base. The time base is established to avoid any slip in thecalculated value of speed. The method of the present invention alsocancels out the switching threshold of the magnetic sensor as indicatedat block 120. This ensures that the time-base is firmly established inthe control algorithm to avoid any slip in calculated values. The slopeof the current waveform in the active phase is monitored to fine-tune afiring angle from the encoder software as indicated at block 122. Theslope of current is evaluated for a fixed duration of time to fine-tunethe firing angle from encoder software. The calculated shaft speed isupdated 30 times in one cycle to continuously track any change in speed.

Based on the estimated time base, the pulse time t_(on) is individuallyadjusted for each phase as indicated at block 124. By this step, pulset_(on) is individually adjusted for each phase, equaling thirtycorrections per mechanical revolution. The method then monitors theshaft speed to track any change in speed and fix the dwell angle basedon the shaft speed as indicated at block 126. The speed can be verytightly monitored, in one instance with as many updates as 30 perrevolution, thereby providing better resolution than several sensorlessapproaches currently in use.

Dwell is fixed based on speed. A current band is established thatreduces dwell if the command current is below the lower band andincreases dwell if the commanded current is above the upper band. If thecommanded current is below a lower band, the dwell angle is reduced andif the commanded current is above an upper band, the dwell angle isincreased. This has the effect of increasing the phase current at lowerpower levels thereby operating the SRM at a higher saturation level. Fora given power output, decreasing dwell will command a lower phasecurrent.

FIG. 3 represents an apparatus 200 for quasi-sensorless control of thehigh rotor pole switched-reluctance motor (HRSRM) comprising aswitched-reluctance motor 202 having a stator and a rotor. The rotorcomprises a plurality of circumferentially spaced rotor poles and isrotationally related to a motor shaft, the motor shaft having a magneticsensor. The HRSRM further comprises a Programmable brushless directcurrent load 204 connected to an output of the switched-reluctance motor202 via an inline torque meter 206 and a converter 208 connected to theload. A software encoder is positioned in the control processor 210, thesoftware encoder establishing a firm time base on the magnetic sensor.The rotor produces an inductance profile in each of the stator poles aseach of the rotor poles comes into and out of alignment with the statorpoles when the rotor is rotated. At least three signals are generatedper rotor pole so that the shaft speed of the motor is calibrated. Athree-phase inverter 212 controlled by the control processor 210 isconnected to the switched-reluctance motor 202. The inverter 212 isadaptable to act as a power supply for the switched-reluctance motor202, the control processor 210 has a software control module and thesoftware encoder.

The quasi-sensorless control of the high rotor pole switched-reluctancemotor (HRSRM) 202 naturally calibrates the control algorithm to theinductance profile of the switched-reluctance motor 202 that is beingtested. The switched-reluctance motor 202 is scalable to all powerlevels and the creation of a control algorithm does not have to becalibrated for all motor specifications and power ratings. Theswitched-reluctance motor 202 can automatically accommodate formotor-to-motor or process variations.

In one embodiment, the system comprises a method for controlling a highrotor pole switched-reluctance motor (HRSRM), the method comprising thesteps of: estimating an initial position of the rotor of the HRSRM usinga unique sequence of relation between the phase inductances of theHRSRM; applying current to an active phase winding to rotate the motor;applying a voltage pulse to an inactive phase winding; measuring currentresponse in the inactive phase; applying multiple diagnostic pulses tothe inactive phase to identify the next phase; establishing a firm timebase for a software control module on a magnetic sensor; updating thetime base by generating a motor index pulse from the magnetic sensor;calculating a shaft speed of the motor and calibrating a softwareencoder to operate on the time base; canceling out a switching thresholdof the magnetic sensor; monitoring a slope of the current waveform inthe active phase to fine-tune a firing angle from the encoder software;adjusting the pulse time t_(on) based on the estimated time base;monitoring the shaft speed to track any change in the speed; andadjusting the dwell angle based on the shaft speed and the current.

Referring to FIGS. 4A-4C, the current waveforms of the three-phase SRMat different loads in accordance with the present invention areillustrated. A current waveform of the three-phase SRM at a light loadwith a speed of 900 RPM and 1 Nm is illustrated in FIG. 4A and thecurrent waveform of the three-phase SRM at a partial load having a speedof 1200 RPM and 6 Nm is illustrated in FIG. 4B. FIG. 4C illustrates thecurrent waveform of the three-phase SRM at full load having a speed of1800 RPM and 6 Nm. The method of the present invention monitors theslope of the current waveform to fine-tune the firing angle from theencoder software. FIGS. 4A-4C illustrate the variation of dwell angle inaccordance with load variations. The processor with the software controlmodule advances or retards the timing of the current waveform inresponse to load or commanded current changes. The amount of advance orretard is chosen to maintain the current slope to a constant referencevalue.

The foregoing description of the preferred embodiment of the presentinvention has been presented for the purpose of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teachings. It is intendedthat the scope of the present invention not be limited by this detaileddescription, but by the claims and the equivalents to the claimsappended hereto.

What is claimed is:
 1. A method for controlling a high rotor poleswitched-reluctance motor (HRSRM), the method comprising the steps of:a) establishing a firm time base for a software control module on amagnetic sensor; b) updating the time base by generating a motor indexpulse from the magnetic sensor; c) calculating a shaft speed of themotor and calibrating a software encoder to operate on the time base; d)canceling out a switching threshold of the magnetic sensor; e)monitoring slope of the current waveform of an active phase winding ofthe motor to fine-tune a firing angle from the encoder software; f)adjusting a pulse time t_(on) based on the estimated time base; g)monitoring the shaft speed to track any change in the shaft speed; andh) adjusting a dwell angle based on the shaft speed and the current. 2.The method of claim 1 wherein the rotor is aligned with respect to theinitial position to start with a known phase.
 3. The method of claim 1wherein the initial position is estimated using a diagnostic pulse on aninactive phase winding during the initial rotation by estimating aninductance profile.
 4. The method of claim 1 wherein the rotor producesthe inductance profile in each of the stator poles as each of the rotorpoles comes into and out of alignment with the stator poles when therotor is rotated.
 5. The method of claim 1 wherein the active phasewinding is the phase that has a rotor pole close to the alignedposition.
 6. The method of claim 1 wherein a pulse of voltage is appliedto the phase winding to measure the current response.
 7. The method ofclaim 1 wherein the multiple diagnostic pulses are applied for at leastten rotations to establish the firm time base for the software controlmodule.
 8. The method of claim 1 wherein after ten rotations thesoftware control module establishes the time base and the inactive phaseis not necessary to maintain operation.
 9. The method of claim 1 whereinat least three signals are generated per rotor pole and the shaft speedof the motor is calibrated.
 10. The method of claim 1 wherein time baseis established to avoid slip in calculated value of speed.
 11. Themethod of claim 1 wherein the dwell angle is reduced if the commandedcurrent is below a lower band.
 12. The method of claim 1 wherein thedwell angle is increased if the commanded current is above an upperband.
 13. A method for quasi-sensorless adaptive control of a high rotorpole switched-reluctance motor (HRSRM), the method comprising the stepsof: a) establishing a firm time base for a software control module on amagnetic sensor; b) updating the time base by generating a motor indexpulse from the magnetic sensor; c) calibrating a shaft speed of themotor and calibrating a software encoder to operate on the time base; d)canceling out a switching threshold of the magnetic sensor; e)monitoring a slope of a current waveform in an active phase winding ofthe motor to fine-tune a firing angle from the encoder software; f)adjusting a pulse time t_(on) based on the estimated time base; and g)monitoring the shaft speed to track any change in speed and fixing adwell angle based on the shaft speed.
 14. The method of claim 13 whereinthe rotor produces an inductance profile in each of the stator poles aseach of the rotor poles comes into and out of alignment with the statorpoles when the rotor is rotated.
 15. The method of claim 13 wherein theactive phase winding is the phase that has a rotor pole close to thealigned position.
 16. The method of claim 13 wherein a pulse of voltageis applied to the phase winding to measure the current response.
 17. Themethod of claim 13 wherein the multiple diagnostic pulses are appliedfor at least ten rotations to establish the firm time base for thesoftware control module.
 18. The method of claim 13 wherein after tenrotations the software control module establishes the time base and theinactive phase is not necessary to maintain operation.
 19. The method ofclaim 13 wherein at least three signals are generated per rotor pole andthe shaft speed of the motor is calibrated.
 20. The method of claim 13wherein time base is established to avoid slip in calculated value ofspeed.
 21. The method of claim 13 wherein the dwell angle is reduced ifthe commanded current is below a lower band.
 22. The method of claim 13wherein the dwell angle is increased if the commanded current is abovean upper band.